Vaccination is one of the best examples of successful manipulation of the immune system. However, vaccination against pathogens like influenza that have a high mutation rate, display variable antigens, and require T cell immunity are still tenuous. The immune system is made up of a diverse collection of cells, each of which has distinct sets of triggers that elicit unique and overlapping responses. Effector, Memory and Tolerant states of CD8 T cells all emerge from a different set of multiple interactions regulated i space and time. Mapping those interactions during natural infection and vaccination and understanding which ones are regulating T cell fate are essential to understand how immune balance is maintained. We hypothesize that specific cell interactions regulate long-term protection and can be manipulated during vaccination. We and others have recently used live imaging to demonstrate that T cell priming takes place in the limited physical volume of the 'reactive' lymph node under highly dynamic conditions that would appear to permit considerable mixing of ongoing responses. A key finding is that there is a 'Critical Differentiation Period' tha coincides with individual activated T cell clones coming together forming a T cell niche environment. We propose to develop a new imaging technique based on fluorescent barcoding of individual T cell clones coupled with organ-clearing to increase our knowledge of the niche-dependent regulation of T cell fate. This method will be the first imaging technique to give access to single-cell behavior over time, as opposed to current methods characterizing the behavior of a population. It will rely on the expression of a unique set of fluorescent proteins in each cell within an immune population of interest. Using whole Lymph Nodes imaging coupled to fluorescent barcoding, we will specifically investigate how T cell niches are formed and for how long they persist. T cell niches are platforms where contact-dependent signals regulate CD8 T cell differentiation and so understanding interactions here will affect multiple disorders. This information will give novel information on CD8 T cell fate that will be critical to enhance vaccination against viruses.